Logging while drilling (LWD) measurements, while initially conceived primarily as alternatives or replacements for similar wireline measurements, are increasingly used to provide information which can influence the drilling process. This has progressed from determining the optimum casing point near an overpressured zone, to using gamma ray or density measurements to place and maintain a borehole within a geologic zone. In the latter case a number of different tools can be used, two examples of which are so-called azimuthal natural gamma ray tools and azimuthal density tools. In each case the tools are sensitive to the differences in the formation around the surface of the borehole as the borehole nears or crosses a formation bed boundary. As the tools rotate in the borehole, the data from them reflect these formation variations, and can be very important in “geo-steering”—that is, in the process of drilling a highly deviated or horizontal well bore through a formation, because the seismic data which are initially used to target a formation are often of insufficient quality and resolution to accurately place the well bore to the satisfaction of the driller. Measurements taken of the formation by tools near the bit in the drilling string provide information which can inform the driller when the target bed has been entered or exited, thereby allowing modifications to the drilling program that will provide much more value and higher success than would be the case using only seismic data.
The two most common methods of drilling horizontal and highly deviated wells are those which use mud motors, e.g., positive displacement motor (PDM), Moineau motor, turbine-type motor and the like, and those employing so-called rotary steerables. In the case of mud motors, the bottom hole assembly consists of (working up from the bottom of the drill string) the drilling bit, a short section of drill pipe or drill collar called a “bent sub”, a mud motor assembly, and a LWD assembly consisting of logging sensors and systems capable of recording data as well as transmitting data to the surface, and finally the remainder of the drill collars and drill pipe. In either case, the logging sensors located in the LWD section, are some tens of feet from the drilling bit because of the length of the mud motor or the rotary steerable. Typically these sensors provide information about the formation that is within a few inches or one or two feet of the sensors themselves. Thus, if the driller is interested in the location of a bed boundary, a length of hole must be drilled approximately equal to the distance between the bit and sensors before the sensors are near enough to the bed boundary to sense its presence and for that information to be transmitted to the driller. “Course corrections,” if desired following receipt of this information, are accomplished only through gradually changing the borehole direction, and thus additional lengths of borehole, equal to two or three times the bit-to-sensor distance, of necessity must be drilled before proper placement of the borehole is achieved. In this manner significant sections of horizontal boreholes intended to be placed in productive zones may be rendered useless. It is of course of great interest to avoid these kinds of drilling errors if possible.
Accordingly, there are at least two approaches that are suggested to increase the efficiency of drilling horizontal boreholes. One is to provide deeper reading sensors in the LWD string. Although the sensors remain a long distance from the bit, the target bed or bed boundary is almost always approached from a shallow angle. If the bed is of large area extent, the deeper reading sensors will be able to “anticipate” the approach of the target before the boundary is crossed by the bit, even though the sensors are significantly behind the bit. The other alternative is to move sensors closer to the drill bit itself, either by placing small units in the short sections of the drill collar between the mud motor or rotary steerable and the bit, or in the bit itself. A disadvantage of the former approach is the difficulty of doing this without designing the sensor directly into the mud motor assembly, which limits the compatibility of the mud motors with other drill string components. Alternatively, designing sensors in the bit presents a similar problem in that constraints are placed on the choice of bits if the sensors are placed there. However, recent developments in rotary steerable technology and more conventional drilling techniques make the latter alternative more attractive.
The bit itself does represent an ideal choice of locations for certain types of sensors: specifically, high resolution sensors which require formation contact and are capable of taking data samples of various parameters representative of the borehole wall and formation properties in the vicinity of the borehole wall, wherein a high resolution image of the formation surrounding the borehole may be produced from these data samples. Innovative drilling products recently introduced which use long gauge bits are ideal candidates for sensors in the bit, as will be indicated below. Example products are the SLICKBORE® system which uses a mud motor and differently designed bit assembly, as well as the rotary-steerable system, e.g., GEO-PILOT® SLICKBORE® and GEO-PILOT®, are registered trademarks of Halliburton Energy Services, Inc,. Beltline Road, Carrollton, Tex. 75006. In each case these drilling tools are designed to produce a much smoother hole than is normally the case.
During drilling with traditional mud motor systems, the drill string is not rotated when the direction of the hole is being changed, and it is rotated when “drilling ahead”. However, because of the bent sub assembly, a “spiral” hole is produced when rotating the string. Even if the placement of the hole is correct, the spiral sections so produced cause problems with placement of casing, and thereby limit the length of producing hole that can be obtained. However, rotary steerable systems are designed to drill “smooth” holes rather than spirals, e.g., SLICKBORE® and GEO-PILOT®.
A necessary aspect of both the bent sub and rotary steerable systems is the use of long gauge bits, where the gauge part of the bit consists of several inches of flutes between the “pin” of the bit and the bit itself. It is possible to place an insert with sensors in this section. Indeed this has been done by Sperry-Sun.
An important wireline capability that is not currently available in LWD is the ability to create a high resolution image of the formation surrounding the borehole. In the case of wireline logging, images are created using data from small, shallow reading sensors which are either in contact with, or in very close proximity to, the borehole wall. These sensors are commonly high resolution ultrasonic transducers or electrode sensors, and their responses are sensitive to small portions of the borehole wall. Images are constructed by assembling the responses of a number of similar sensors distributed around the borehole, as in the case of tools using a number of small electrodes on a pad which is forced against the borehole wall while logging. Alternatively, the image may be created from a large number of individual measurements taken rapidly by a single sensor rotating in the borehole. In contrast with wireline logging, reliable wall contact is usually not available in LWD logging. Although sensors that require wall contact are available in the LWD logging—density tools are an example—wall contact is intermittent, and a great deal of effort is expended to compensate for this fact. Images are produced with such tools, but these are necessarily of low resolution because of the physics of the sensor and the lack of reliable contact with the borehole wall. However, lack of wall contact is much less of an issue for long gauge bits, where the purpose of the extend gauge is to maintain contact with the borehole wall in order to have axial alignment of the bit with the borehole. Sensors placed in these long gauge bits will have the advantage of an environment where the wall contact is constant, or where the standoff from the wall is minimal. Thus, the long (extended) gauge bit represents “prime real estate” for the purpose of obtaining data samples sufficient to create high resolution images.